The Cloud Chamber

The cloud chamber is one of the most basic tools for investigating particle physics. The cloud chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supercooled, supersaturated water or alcohol vapour formed when a piston is pulled down, causing the gas in the chamber to expand and cool below the point of condensation.

When an alpha particle or beta particle from the source interacts with the mixture, it ionizes it. The resulting ion act as condensation nuclei, around which a mist will form (because the mixture is supersaturaed). The high energies of alpha and beta particles mean that a trail is left, due to many collisions of the particle with air molecules

Ions are produced along the path of the charged particle. These tracks have distinctive shapes (for example, an alpha particle's track is broad and straight, while an electron's is thinner and shows more evidence of deflection by collisions). If a uniform magnetic field is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the Lorentz force law. Lighter particles will execute a circle of smaller radius.